Control and power supply wiring on commercial and military aircraft and vehicles is provided by low voltage cables that are typically insulated with PTFE (polytetrafluoroethylene) or polyimide, sometimes referred to by the trade names Hypolon™ or Mylar™ insulation. PTFE offers high temperature resistance, high dielectric strength, fluid resistance and low smoke generation. Its weaknesses include susceptibility to cold flow and low dynamic cut-through resistance. Polyimide has all of the properties of PTFE but has improved resistance to dynamic cut-through. Unfortunately polyimide has been found to be susceptible to arc propagation and degradation due to hydrolysis.
Such control cables typically consist of central solid or stranded conductors of copper or aluminum with an insulating layer, twisted in pairs, usually in bundles of 36 pairs or more and protected by a jacket of tough polymer material to prevent abrasive or mechanical damage during installation, maintenance and operation of the aircraft. The insulation of such control cables has a typical life time of about 30 to 40 years. However, various factors can lead to premature degradation of the insulation resulting in the deterioration of the electrical and physical properties of the insulating material and to eventual failure of the cable. These factors can include loss of plasticizer with time and temperature, hydrolysis, cold flow, filamentary alignment of the insulation fibres making any insulation tapes susceptible to cracking, unravelling of taped insulation with age, and loss of dielectric, chemical and mechanical properties due to temperature cycling and high temperature operation. Deterioration of the insulation mechanical properties can be so extensive that cracking and opening of the insulation material can occur. Many older commercial and military aircraft are nearing the end of their service lives but are still operating with possible suspect wiring that could lead to major problems without any warning.
At 400 Hz operation, the normal voltage frequency in these vehicles, sparking to the grounded frame or fuselage and between wires can lead to the catastrophic failure of the control wiring and/or the power supply to electrical servo-motors used to control the plane engines and the flight system. Eventually fire can result in the wiring with complete loss of control of the aircraft. Smoke is a by-product of the event placing personnel at risk.
It is important therefore for aircraft operators to be able to test power and control cables for degradation to allow replacement prior to failure and to permit an orderly replacement schedule. Preferably such testing would be done “live”, that is without de-energising the cable prior to testing. There is therefore a need for a diagnostic method which permits on-line, non-destructive live diagnostics in aircraft wiring.